The Indus or Harappan Civilization was one of the greatest societies in the ancient world. Contemporary with Ancient Egypt and Mesopotamia, its population is thought to have made up 10% of the world at one point. Yet, between 3900 and 3000 years ago, the cities crumbled and the people disappeared. What went wrong, and what can we learn from its demise?

A recent, major interdisciplinary undertaking has delved into the geology of the region in this period, and it seems that this major civilization sprung up in a perfect climactic window — which then closed, bringing about its eventual downfall.

The Indus Valley at this point was in the process of gradually declining monsoons. These once-unstoppable torrents of water had rendered the area uninhabitable for permanent settlements. But as the monsoons reduced, they left in their wake ultra-fertile soil. For 2,000 years, the ensuing society relied on crop surpluses brought about by manageable monsoons, and the rich soils they brought.

However, the rains continued to lessen, and these monsoon floods became unreliable. This caused a shift away from the large cities and the surpluses that the Harappans needed to continue, and instead people migrated Eastward.

Dorian Fuller of University College London said in a press release:

"We can envision that this eastern shift involved a change to more localized forms of economy: smaller communities supported by local rain-fed farming and dwindling streams. This may have produced smaller surpluses, and would not have supported large cities, but would have been reliable...thus cities collapsed, but smaller agricultural communities were sustainable and flourished. Many of the urban arts, such as writing, faded away, but agriculture continued and actually diversified."

By combining millennia-old soil samples, satellite photos and topographic data collected by the Shuttle Radar Topography Mission (SRTM), and archaeological data, the researchers unearthed what caused the collapse of an immense and sophisticated urban culture thousands of years ago.